Ultra-high performance concrete reinforcement bars

ABSTRACT

The ultra-high performance concrete reinforcement bars replace conventional steel tension bars and the like in the reinforcement of construction elements, such as concrete beams. The ultra-high performance concrete reinforcement bars are formed from ultra-high performance concrete reinforced with steel fiber. The reinforcement bars are preferably deformed to improve bonding at the interface between conventional concrete in the beam and the reinforcement bars. The bars are disposed in the area of the beam known as the tension area when the beam is subjected to a bending stress to provide tensile reinforcement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to reinforcement bars for concreteconstruction, and particularly to ultra-high performance concrete barsthat are reinforced with steel fiber and used for tension reinforcement.

2. Description of the Related Art

Recently, ultra-high-performance concrete (UHPC) was developed byagencies as a new generation of cementitious material. UHPC ischaracterized by being a fiber-reinforced cement composite material withcompressive strengths in excess of 150 MPa and flexural tensile strengthexceeding 30 MPa. UHPC is also characterized by its constituent materialmake-up, typically including fine-grained sand, silica fume, reinforcingmaterials and special blends of high-strength Portland cement. It shouldbe noted that UHPC contains no large aggregate. The current types ofUHPC in production differ from normal concrete in compression by theirstrain hardening, followed by sudden brittle failure. Ongoing researchinto UHPC failure due to tensile and shear failure is being conducted bymultiple government agencies and universities around the world.

In spite of its advantages, UHPC is not widely used. The only commercialsource of a proprietary mix for UHPC widely available in the UnitedStates is Ductal, produced by LaFarge Company of France, although othercommercial UHPC mixes are commercially available outside the UnitedStates. Thus, most concrete construction continues to use conventioncement mixes. Nevertheless, it would be desirable to provide aconvenient way for concrete construction to take advantage of thesuperior properties of UHPC concrete material.

Thus, ultra-high performance concrete reinforcement bars solving theaforementioned problems is desired.

SUMMARY OF THE INVENTION

The Ultra-High Performance Concrete (UHPC) reinforcement bars areprecast concrete bars made from UHPC that can be used as tensionreinforcement in concrete construction elements, replacing steelreinforcement bars. The UHPC bars may be made from any known UHPC mixhaving steel fibers dispersed through the mix. The UHPC mix willtypically contain a mixture of cement, micro-silica, fine sand, steelfiber, and a superplasticizer. The UPHC is mixed with water, cast in amold, and cured into precast bars having suitable cross-sectionaldimensions and length for providing reinforcement to beams and otherconcrete construction elements made from conventional concrete, therebyforming hybrid or composite concrete elements. Although the UHPCreinforcement bars may have smooth faces, preferably the UHPCreinforcement bars are deformed bars, having deformations cast inopposite lateral faces of the bar to provide good bonding between thereinforcement bars and the conventional concrete.

These and other features of the present invention will become readilyapparent upon further review of the following specification anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial side view in section of an exemplary ultra-highperformance concrete reinforcement bar according to the presentinvention.

FIG. 2 is an end view of a hybrid concrete beam having the tensionportion of the beam reinforced with a pair of UHPC reinforcement barsaccording to the present invention.

FIG. 3 is a load-deflection plot for a first sample of three identicalconcrete beams (150×150×760 mm) reinforced with a pair (25×50 mm each)of the ultra-high performance concrete reinforcement bars according tothe present invention.

FIG. 4 is a load-deflection plot for a second sample of three identicalconcrete beams (50×200×900 mm) reinforced with a pair (25×25 mm each) ofthe ultra-high performance concrete reinforcement bars according to thepresent invention.

FIG. 5 is a plan view of an embodiment of a UHPC reinforcement baraccording to the present invention having deformations formed onopposing lateral faces of the bar.

Similar reference characters denote corresponding features consistentlythroughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ultra-high performance concrete reinforcement bars are precastconcrete member made from ultra-high performance concrete (UHPC) thatare dimensioned and configured for use as reinforcing material forconstruction elements made from conventional concrete, such as concretebeams, replacing conventional steel tension reinforcements and the liketo form hybrid or composite construction elements. Although the UHPCreinforcement bars may have smooth faces, preferably the reinforcementbars are deformed to provide good bonding between the reinforcement barsand the conventional concrete used to form the beam or other concreteconstruction element.

Conventional concrete is generally formed from cement, water, andaggregate material (such as sand, stones, and gravel), which includesboth coarse and fine aggregate material. Concrete is generally strong incompression, but its tensile strength is low enough that the concretemay require reinforcement with steel bars, which are often deformed byprojections or lugs to improve bonding between the concrete and thesteel reinforcement bars to prevent sliding at the interface between thetwo materials.

In recent years, ultra-high performance concrete has been developed thathas improved compressive strength, and when reinforced with steelfibers, possesses sufficient tensile or flexural strength that it may beused, often without the need for reinforcement with steel bars. Inaddition, the material has greater durability than conventional concreteand may be used in environments where conventional reinforced concretewould be subject to weakening by corrosion of the steel reinforcementbars.

The term “ultra-high performance concrete” does not have a standarddefinition. A technical note published by the U.S. Federal HighwayAdministration in March 2011 (FWHA Publication No. FHWA-HRT-11-038)defines UHPC as “a cementitious material composed of granularconstituents, a water-to-cementitious material ratio less than 0.25, anda high percentage of discontinuous internal fiber reinforcement. Themechanical properties of UHPC include compressive strength greater than21.7 ksi (150 MPa) and sustained postcracking tensile strength greaterthan 0.72 ksi (5 MPa). UHPC has a discontinuous pore structure thatreduces liquid ingress, significantly enhancing durability as comparedto conventional and high-performance concretes.” The aggregate materialin UHPC generally has a particles size less than 1 mm, i.e., there is anabsence of coarse aggregate, generally permitting the material to bemore tightly packed, requiring less water and producing smaller pores,improving compressive strength compared to conventional concrete. Asused herein, the term “ultra-high performance concrete” refers toconcrete thus defined, in which the fiber reinforcement comprises steelfibers, while the term “conventional concrete” refers to concrete havingcoarse aggregate and a water-to-cementitious material greater than 0.25.

In order to test the viability of using UHPC reinforcement bars toprovide tension reinforcement for hybrid concrete construction elements,the inventors tested the concept experimentally as follows. The mixdesign used for UHPC bars was developed in-house using local materials.It comprised (per cubic meter of concrete): ASTM Type I Portland cement900 kg, micro-silica 220 kg, fine sand 1005 kg, steel fiber 157 kg(about 6.2% by weight of the UHPC mix), superplasticizer (Glenium 51)(an admixture that improves workability of the UHPC), and water 162.4 kg(representing water-binder ratio of 0.145). The steel fibers were 0.15mm in diameter and 12.7 mm in length, and had ultimate tensile strengthof 2500 MPa. The UHPC bars, rectangular or square in section, are castin a wooden mold with built-in vertical grooves to provide thedeformation to the UHPC bars. After mixing the materials in a horizontalrevolving planetary mixer, the mixture was placed in the mold, leavingthe top surface of the bars unfinished. After a few hours, the bars arede-molded and then thermal-cured for two days at 90° C., as it was foundthat heat-curing enables the bars to rapidly gain strength, which allowsavoiding longer periods of moist-curing.

FIG. 1 shows a side view in section of a deformed UHPC reinforcement bar10, including the steel fibers 14 dispersed in the matrix of UHPCconcrete 12. FIG. 5 shows a top view of the reinforcement bar 10,showing the deformations (or deformities) comprising shallow vertical(orthogonal to the length of the bar 10) projections 20 spaced apartalong opposing sides of the bar 10. For a reinforcement bar having awidth of 25-50 mm, the projections 20 may extend outward from the bar 10about 3-4 mm and may be spaced apart about 100 mm along the length ofthe bar 10, and extend in length from the top face of the bar 10 to thebottom face of the bar 10, i.e., for the height of the bar 10 as viewedfrom the side of the bar 10. It will be understood that the deformationsmay have other cross-sectional shapes or other orientations, so long asthe deformations provide sufficient surface area to improve bonding atthe interface between the UHPC reinforcement bars 10 and theconventional concrete of the hybrid concrete beam or constructionelement. For example, the projections 20 may have a rounded end face orbe triangular in cross section, and the projections may be orienteddiagonally or obliquely on the lateral faces of the bar 10. A typicalUHPC bar 10 has flexural tensile strength of about 32 MPa and directtensile strength of about 11-12 MPa.

The experimental program involved testing a series of beam specimens of150×150×760 mm and 150×200×900 mm in size. Each normal or conventionalconcrete beam was reinforced with two UHPC bars 10 of rectangular orsquare cross section having transverse dimensions of 25×25 mm, 25×50 mmand 50×50 mm, and an axial length equal to the length of the beam. Thebeams were moist cured for 28 days prior to testing in a four-point bendtest. The failure load, mid-span deflection and the mode of failure wasrecorded for each test.

FIG. 2 shows an end view of a hybrid concrete beam 16 having a pair ofthe UHPC reinforcement bars 10 embedded in the beam made of conventionalconcrete 18 in parallel along the bottom face of the beam 16. Generally,the top face of the beam 16 is the load bearing face and defines acompression area that extends downward from the top face to a neutralaxis where the beam experiences substantially zero bending stress. Theregion extending from the bottom face to the neutral axis defines atension area. When a beam is subjected to a bending stress, the beamdeflects because its compressive side (adjacent the top face) shortens,and its tension side (adjacent the bottom face) lengthens, which mayproduce cracks and ultimate failure of the beam. Consequently, in thehybrid beam 16, the UHPC reinforcement bars 10 are embedded in thetension area of the beam 16, along the bottom face of the beam 16, asshown in FIG. 2.

Results show that the failure occurs with the development of cracks inthe UHPC bars 10 within the maximum moment region. It should be notedthat the behavior and mode of failure of UHPC reinforced beams isdifferent from that of concrete beams reinforced with steel bars. A UHPCreinforced beam 16 attains its peak load at the development of firstflexure cracking, following which a declining load path follows withincreased deflection, exhibiting a gradual softening mode of failure.The peak load corresponds to the attainment of maximum flexural stresswithin the range of 28-32 MPa, calculated using transformed sectionproperties. No bond-slip problem was noted, and all beams 16 failed inflexure with the development and propagation of a single vertical crack.The deformed shape of the precast bars 10, along with the rough topsurface (from leaving the top of the mold open), enables the bars 10 todevelop full strength without any bond-slip.

The experimental findings have clearly established the possibility ofutilizing the precast UHPC bars 10 in one-way slab-type construction,for which shear is not a critical design consideration. For flexuraldesign, a normal concrete member can be reinforced with deformed UHPCbars, replacing mild steel bars. The approximate amount of UHPC bars 10that can be used to replace mild steel bars is about 15-20 times thearea of steel bars. As the cross section of a UHPC bar 10 is muchlarger, the increased area of UHPC bars 10 can be accommodated withinthe beam section.

For precast one-way slabs, the use of precast UHPC bars 10 appears to befeasible and attractive.

FIG. 3 shows a typical load-deflection plots of three identical beamspecimens (beams of 150×150×760 mm reinforced with 2 UHPC bars of 25×50mm in cross section). The peak load varied between 35 to 40 kN. Thepost-cracking behavior shows increased deflection with decline in load.At about 50% of the peak load, the mid-span deflection increased toalmost twice the value at the peak load. In FIG. 4, the load-deflectionplot is shown for three identical UHPC-reinforced beam specimens of150×200×900 mm, reinforced with 2 UHPC bars of 25×25 mm in crosssection. The computed flexural tensile strength corresponding to thepeak load varied from 26 to 31 MPa. It should be noted that a UHPC bar10 does not have much post-cracking elongation in direct tension, unlikesteel bars. In flexure, the progressive crack-growth allows UHPC to showa softening mode of failure (FIGS. 3 and 4).

Thus, the ultra-high performance concrete reinforcement bars may providetension reinforcement for conventional concrete in hybrid or compositeconcrete beams or construction elements.

It is to be understood that the present invention is not limited to theembodiments described above, but encompasses any and all embodimentswithin the scope of the following claims.

1. An ultra-high performance concrete reinforcement bar, comprising anelongate bar made from ultra-high performance concrete havingdiscontinuous steel reinforcing fibers, the elongate bar beingdimensioned and configured for providing tension reinforcement for aconventional concrete construction element, wherein the transverse crosssectional dimensions are in the range of approximately 1 inch to 2inches.
 2. The ultra-high performance concrete reinforcement baraccording to claim 1, wherein the elongate bar is deformed along thelength of the elongate bar. 3-6. (canceled)
 7. The ultra-highperformance concrete reinforcement bar according to claim 2, wherein thetop face of the elongate bar is rough for further improving bondingbetween the elongate bar and the conventional concrete of theconventional concrete construction element.
 8. The ultra-highperformance concrete reinforcement bar according to claim 1, wherein theelongate bar is square in transverse cross section. 9-13. (canceled) 14.The ultra-high performance concrete reinforcement bar according to claim1, wherein said steel reinforcing fibers have a diameter of about 0.15mm, a length of about 12.7 mm, and an ultimate tensile strength of about2500 MPa.
 15. The ultra-high performance concrete reinforcement baraccording to claim 1, wherein said steel reinforcing fibers compriseabout 6.2% by weight of the ultra-high performance concrete.
 16. Theultra-high performance concrete reinforcement bar according to claim 1,wherein said ultra-high performance concrete has a water-to-cementitiousmaterial ratio of about 0.145.
 17. The ultra-high performance concretereinforcement bar according to claim 1, wherein said elongate bar hasflexural tensile strength of about 32 MPa and direct tensile strength ofabout 11-12 MPa.
 18. In combination, a hybrid concrete constructionelement having a conventional concrete component and an ultra-highperformance concrete reinforcement bar component, comprising: anelongate concrete construction element made from conventional concrete,the construction element having a compression area and a tension areawhen subjected to a bending stress, wherein each of the compression andtension areas includes an elongated compression face and tension face,respectively; and at least one elongate reinforcement bar made fromultra-high performance concrete having steel reinforcing fibers, the atleast one elongate reinforcement bar being incorporated into andextending along the tension face, thereby providing the tensionreinforcement for the construction element.
 19. (canceled)
 20. Thecombination according to claim 18, wherein the at least one elongatereinforcement bar is deformed along its length.
 21. The combinationaccording to claim 18, wherein the at least one elongate reinforcementbar comprises a pair of ultra-high performance concrete reinforcementbars extending in parallel along the length of the tension face of theconstruction element.
 22. In combination, a hybrid concrete constructionelement having a conventional concrete component and an ultra-highperformance concrete reinforcement bar component, comprising: anelongate concrete construction element made from conventional concrete,the construction element having a compression area and a tension areawhen subjected to a bending stress; and at least one elongated, deformedreinforcement bar made from ultra-high performance concrete having steelreinforcing fibers, the at least one elongate, deformed reinforcementbar being incorporated into and extending along the tension area therebyproviding the tension reinforcement for the construction element.